I have a 700r4 behind a 350 in a K2500. It has 3.42 gears if I remember correctly, and I notice it shifts down into direct a lot on almost any little incline.
Its my most frequent alternate commuter over another 350 with 3.08 gears that makes the same trip in overdrive (manual so I control what its doing) and never seems to strain unless ol' lead foot decides to pass. The one with 3.08 gears weighs about 1,000 pounds less than the K2500.
My 270 powered GMC Suburban with 3.08 gears and no overdrive makes the same trip at similiar speeds with no strain (but no passing), and I suspect would behave similiar to the K2500 if it had a 700r4 instead of a SM420, dropping in and out of OD as it needed.
My point is that the 700r4 would give you the benefits of overdrive while dropping into direct or lower when conditions made it appropriate. Unless you are carrying heavy loads or pulling a trailer I'd assume the existing gears will be fine until experience tells you otherwise. With a 12-bolt already in place you are probably going to change the gears rather than the axle, so changing later won't mean changing anything you did during the trans swap.
